Electrical-motor-driven devices have found use in a wide variety of applications. Many of these applications require regular monitoring to ensure that the motor is properly functioning. MCSA is an effective method for efficiently,
reliably and non-intrusively monitoring the condition of electrical-motor-driven devices. MCSA permits individuals to determine the operating condition of rotating equipment. The operating condition can then be related to the maintenance needs of the equipment.
MCSA is based upon the transduction of signals back to the power line via an electrical motor. For example, mechanical vibrations in alternating current (AC) rotating equipment are transduced back to the power line via the electrical motor. Additionally, electrical characteristics of the motor are also transduced back to the power lines. The transduced signals modulate the power line current.
U.S. Pat. No. 4,965,513 to Haynes et al. and U.S. Pat. No. 4,978,909 to Hendrix et al., are examples of the use of motor current signals in assessing the condition of a motor. Both of these patents teach the conditioning and analysis of a motor current signal. As shown in the aforementioned Haynes et al. and Hendrix et al. patents, the signals necessary for MCSA are typically monitored by attaching a current transformer to a lead of the electrical motor. Once the signals are gathered they are conditioned, sampled and analyzed in the frequency-domain with a discrete Fourier transform (DFT). Any periodic time-domain vibrations and fault data produced by the motor are displayed as peaks in the frequency spectra.
The large signal produced by the AC power line current, and its harmonics, is also displayed
in the frequency spectra at a magnitude that can be several orders of magnitude greater than the signals of interest. The spectra of motor current data for these large power line frequencies are expansive and Gaussian in nature when they are sampled by conventional methods. As a result, any anomalies having a frequency near the frequency of the AC power line current are difficult to evaluate and precisely define.
Another major difficulty in existing MCSA techniques is analyzing electrical current frequency information across a wide bandwidth that is already contaminated with 60 Hz and 60 Hz multiples (harmonics) that are always present in AC power supplied to residential, commercial, and industrial users. Power lines in many locations may also contain frequency noise components that are induced in the power lines by other large motor-driven devices. The presence of these power line noise frequencies can complicate and obstruct attempts to analyze electrical current signals.
Another problem associated with prior MCSA techniques is, in some cases, the inability to effectively detect and analyze modulations (e.g., electrical current modulations from a motor-driven device) that are greater than the carrier frequency (generally the AC power line current having a frequency of 60 Hz). An examination of raw motor current signals from an electrically powered device, having modulating mechanical and electrical loads, reveals that each power line harmonic is modulated. These modulated harmonics show up as modulation sidebands appearing on opposite sides of each power line harmonic. When modulations of 30 Hz and greater are present, these sidebands overlap. This adds considerable difficulty to analyzing raw current frequency spectra and processed signals. For example, a 40 Hz modulation creates the following sidebands:
______________________________________ from 60 Hz: 60 - 40 = 20 Hz 60 + 40 = 100 Hz from 120 Hz: 120 - 40 = 80 Hz 120 + 40 = 160 Hz from 180 Hz: 180 - 40 = 140 Hz 180 + 40 = 220 Hz from N .times. 60 Hz: (N .times. 60) - 40 (N .times. 60) + 40 ______________________________________
As a result of the sidebands shown above, it can be difficult to determine whether an observed electric current frequency component is an upper sideband or a lower sideband, and of what power line harmonic.
Attempts have been made to overcome many of the problems associated with MCSA. For example, Soviet Certificate of Invention No. 1,420,555 describes analyzing an injected current for high reliability fault detection. Additionally, U.S. Pat. No. 4,224,652 to Fiorentzis describes a method and apparatus for detecting ground shorts in the rotor circuit of a generator. The patent describes the delivery of a test signal to the electrical network containing the generator's rotor circuit. As a result, a response signal is produced which is indicative of the ground short derived from the network. The test signal utilized in accomplishing the method may be at the network frequency.
Examples of other techniques for monitoring electrical motor conditions are disclosed in U.S. Pat. No. 4,884,023 to Schmidt et al. (use of an impressed current to determine the rotor resistance of a rotating field machine), U.S. Pat. No. 5,153,506 to Trenkler et al. (an AC reference voltage source is used to determine the winding temperature of electric machines), and U.S. Pat. No. 4,689,546 to Stephens et al. (use of current signals by an electrical generator control system).
A continuing need exists for a monitoring apparatus and method which provides efficient, convenient, and reliable determination of the operating condition of machinery, such as electrical-motor-driven devices.